In a flexible interconnect substrate usable for a wearable terminal or the like, a resin material having a low modulus of elasticity and a high coefficient of elongation may be used as a material for an insulating layer that covers a conductive layer. In such an interconnect substrate, interconnect patterns are formed on the insulating layer, and are electrically coupled to the conductive layer through via interconnects disposed inside via holes that are formed through the insulating layer. The via holes may be made by a laser, for example.
A laser generates a large amount of thermal energy when forming the via holes through the insulating layer. When a resin material having a low modulus of elasticity and a high coefficient of elongation is used as the material of the insulating layer covering joint points between the via interconnects and the conductive layer, the stress caused by a difference in thermal expansion coefficients between the conductive layer and the insulating layer is not sufficiently mitigated, resulting in the reduction of adhesion between the conductive layer and the insulating layer. Further, the resin having a low modulus of elasticity and a high coefficient of elongation may be damaged by the thermal energy of laser, resulting in the reduction of adhesion between the conductive layer and the insulating layer.
The reduction of adhesion between the conductive layer and the insulating layer may increase a risk of haloing (i.e., fracture or delaminating resulting from mechanical causes) at the desmear process performed after the forming of via holes with a laser. Haloing causes a short fault between adjacent via interconnects, which reduces the reliability of connections between the interconnect patterns of an interconnect substrate.
Accordingly, there may be a need to provide an interconnect substrate for which haloing is less likely to occur.